This invention pertains to charged-particle-beam (CPB, e.g., electron-beam) microlithography systems. More specifically, the invention pertains to methods and apparatus for adjusting the charged particle beam used in such systems, especially where contrast is obtained in a projected image according to the degree of scattering of the beam from a reticle in which a one-shot transfer field is relatively large, and a large proportion of the beam illuminating the reticle passes with little absorption through the entire reticle.
Electron-beam (as a representative charged particle beam) microlithography systems are attracting greater attention for use in the manufacture of semiconductor devices. Currently, practical use of such systems is mainly limited to developing prototypes of semiconductor devices (e.g., integrated circuits) and for making small production runs of specialized and/or custom devices.
In the earliest electron-beam microlithography systems, the beam is narrowed to a fine point and is scanned in a manner by which the pattern is traced line-by-line (i.e., xe2x80x9cwrittenxe2x80x9d) on the surface of a substrate. These systems are termed xe2x80x9cspot-beam scanningxe2x80x9d systems. Such systems exhibit extremely low xe2x80x9cthroughputxe2x80x9d (i.e., number of wafers that can be processed per unit time).
More recent electron-beam systems employ a xe2x80x9cvariable-shapedxe2x80x9d beam and tend to exhibit higher throughput. In such systems, the transverse dimensions of the beam are larger than in a spot-beam scanning system. In addition, the transverse profile and area of the beam can be changed to some extent in a variable-shaped-beam system.
Other conventional electron-beam systems, termed xe2x80x9ccell-projectionxe2x80x9d systems, are typically used whenever the pattern to be xe2x80x9ctransferredxe2x80x9d to the substrate comprises a relatively large area in which a particular small portion of the pattern is repeated many times (such as in a pattern for a memory chip comprising a large number of identical memory cells wherein each memory cell represents the repeated small portion). The highly repeated portion of the pattern is represented by a cell (approximately 5 xcexcmxc3x975 xcexcm on the substrate) that is exposed multiple times on different respective regions of the substrate.
Yet another conventional approach involves dividing the reticle pattern into multiple xe2x80x9cexposure unitsxe2x80x9d or xe2x80x9csubfieldsxe2x80x9d each defining a respective portion of the overall pattern. Such a reticle is termed a xe2x80x9cdividedxe2x80x9d or xe2x80x9csegmentedxe2x80x9d reticle. The exposure units are exposed individually in an ordered manner using an illumination-optical system located upstream of the reticle and a projection-optical system located between the reticle and the substrate. Such a system is termed a xe2x80x9cdivided-patternxe2x80x9d projection-transfer system. As the exposure units are imaged on the substrate, they are xe2x80x9cstitchedxe2x80x9d together in the proper order to form, after all the exposure units have been exposed, the entire pattern on the substrate.
In cell-projection systems, so-called xe2x80x9cabsorption-stencil reticlesxe2x80x9d are generally used. In such reticles, pattern features are represented as corresponding cutouts formed in and extending through the thickness dimension of a relatively thick (normally about 20 xcexcm thick) silicon membrane. When an xe2x80x9cillumination beamxe2x80x9d impinges on such a reticle, portions of the beam passing through the reticle form a xe2x80x9cpatterned beamxe2x80x9d that propagates downstream away from the reticle. To produce the pattern in the patterned beam, portions of the illumination beam pass through the cutouts (in the same manner as light through a window) and experience little to no scattering or absorption. Other portions of the illumination beam impinge on the non-cutout portions of the reticle (i.e., on the membrane) and are thereby absorbed. Absorption-stencil reticles are also used in divided-pattern projection-transfer systems.
To increase throughput, various schemes for increasing beam current have been investigated. However, with substantial increases in beam current, absorption-stencil reticles are impractical because electrons absorbed by the reticle membrane caused heating of the reticle. Such heating caused major problems with thermal expansion of the reticle. To solve this problem, xe2x80x9cscattering-stencil reticlesxe2x80x9d were proposed.
In a scattering-stencil reticle, most of the electrons impinging on the reticle membrane are transmitted through the reticle rather than absorbed by the membrane. However, such electrons tend to be significantly scattered or diffused as they pass through the membrane. In a scattering-stencil reticle, as in an absorption stencil reticle, electrons passing through the cutouts are not scattered. Image contrast is obtained by placing a contrast aperture (that blocks electrons scattered by the reticle to prevent such electrons from propagating to the substrate) at or near a beam-convergence plane of the projection-optical system. I.e., the contrast aperture is placed at the Fourier plane, in the projection-optical system, of the reticle plane. Thus, scattered electrons that would otherwise impair image contrast are prevented from propagating to the image on the substrate.
With a stencil reticle, an island-shaped membrane feature cannot be disposed at the center of a cutout in the surrounding membrane because the island-shaped membrane feature would have no physical support. This is termed the xe2x80x9cdonut-featurexe2x80x9d problem. To solve this problem, at least the surrounding cutout is split between two xe2x80x9ccomplementaryxe2x80x9d exposure units of the reticle. Each split portion is separately projected and exposed onto the substrate. During exposure of the two exposure units, each split portion is positioned on the substrate such that the two cutouts are stitched together to form the exposed region surrounding the island-shaped non-exposed region. This method is termed dividing a reticle pattern into complementary pattern portions. Unfortunately, dividing a reticle pattern into complementary pattern portions requires two separate exposures to image the pattern portion including the island-shaped feature. The need to perform two exposures rather than one decreases throughput by a corresponding amount.
To solve the donut-feature problem, a xe2x80x9cscattering-membrane reticlexe2x80x9d can be used that comprises a relatively thin (e.g., 2 xcexcm thick) electron-transmissive reticle membrane without cutouts. Pattern features are defined on such a reticle by a corresponding pattern of an electron-scattering material layered on the membrane. As electrons of the illumination beam pass through the membrane, virtually no electron scattering occurs. However, passage of electrons of the illumination beam through the electron-scattering material causes substantial electron scattering. A scattering membrane reticle improves throughput because island features can be projected onto the substrate without the need for complementary exposure units.
In the electron-optical system (comprising the illumination-optical system and the projection-optical system) of a conventional electron-beam microlithography apparatus, it is necessary occasionally to adjust the axial alignment, focal position, and/or the amount of astigmatism correction exhibited by the electron-optical system. For axial alignment, the respective excitation currents or voltages applied to the projection lenses and/or deflectors are set to xe2x80x9cstandard conditionsxe2x80x9d that cause the electron beam to pass through the center of the contrast aperture. Such an axial alignment is typically performed whenever, for example, an exposure-pattern lot is changed or whenever periodic adjustments are made to the microlithography apparatus.
In spot-beam scanning systems, since the beam diameter is very small at less than 1 xcexcm, beam axial alignment can be performed using methods as used in scanning electron microscopy. For instance, the beam is scanned over a reference plane (represented by the reticle surface or the substrate surface) and the location of the beam axis is accurately determined by detecting and analyzing signals created by electrons in the scanning beam (e.g., absorption-current signal, backscattered-electron signal, secondary electron signal, etc.). Similar methods can be employed in variable-shaped-beam systems in which the beam diameter can be adjusted down to a very small diameter.
A cell-projection system can employ a variable-shaped beam, especially to form on the substrate portions of the pattern that are not repeated. Whenever an absorption stencil reticle is used with such a system, axial alignment can be performed in a manner substantially identical to that used in a spot-beam scanning system. This is because a spot beam can be formed using the variable-shaped beam system.
However, with a divided-pattern projection-transfer system, the beam dimension is relatively large at (1 mm)2 at the mask and (0.25 mm)2 at the substrate. Such a wide beam causes excessive dispersion of signals that would otherwise be useful for axial alignment of the beam, such as a backscattered-electron signal from an axial alignment pattern, for reliable use in axially aligning the beam. As a result, in contrast to axial alignment methods as used in spot-beam scanning systems or variable-shaped beam systems, the signal is inadequate for use in making sufficiently accurate determinations of beam axial alignment.
In addition, whenever a scattering reticle is used, such as a scattering-stencil reticle or a scattering-membrane reticle, the center of the beam cannot be found by conventional methods. This is because some of the highly scattered particles can pass through the contrast aperture together with particles that are less scattered, making it difficult to perform axial alignment.
Divided-pattern projection-transfer systems normally do not include a mechanism for shaping the beam in a variable manner; therefore, the beam cannot be stopped down sufficiently for axial alignment. Also, beam diameter conventionally cannot be decreased using a reticle because the resulting beam-current density would be too low to provide a sufficient signal-to-noise (S/N) ratio in the beam-reflection signal.
The present invention addresses the shortcomings of conventional methods as summarized above. An object of the invention is to provide adjustment methods for electron-beam and other charged-particle-beam (CPB) microlithographic exposure apparatus that are suitable for performing axial and other alignments of the optical systems of such apparatus.
According to one aspect of the invention, methods are provided for performing a beam alignment in a CPB microlithography apparatus utilizing a segmented scattering-stencil reticle. The apparatus includes an illumination-optical system that illuminates a region of a segmented reticle defining a pattern of features to be transferred to a sensitive substrate using an illumination beam. The apparatus also includes a projection-lens system that projects a patterned beam, formed by passage of the illumination beam through the illuminated region of the reticle, carrying an image of the illuminated region onto the sensitive substrate. In a representative embodiment of the method, a segmented scattering-stencil reticle is provided in which the features are defined by corresponding cutouts in a reticle membrane that transmits the illumination beam but scatters particles in the illumination beam as the illumination beam passes through the reticle membrane. The reticle comprises a xe2x80x9cblackxe2x80x9d subfield and a xe2x80x9cwhitexe2x80x9d subfield each sized similarly to an exposure unit of the reticle. The reticle is placed at an axial location at which the black and white subfields can be individually illuminated by the illumination beam in a manner similarly to illumination of an exposure unit of the reticle by the illumination beam. A contrast aperture, serving to block passage therethrough of charged particles of the patterned beam scattered by passage through the reticle, is placed substantially at a beam-convergence plane of the projection lens. (Such a plane is also the Fourier plane of a plane defined by the reticle.) The contrast aperture prevents the scattered particles from propagating to the sensitive substrate. The white and black subfields are selectively illuminated individually as required with the illumination beam to align components of the illumination-optical system and projection-lens system.
According to another representative embodiment of methods according to the invention, a segmented scattering-membrane reticle is provided in which the features are defined by corresponding regions of a material that highly scatters charged particles of an illumination beam passing therethrough. The material is layered on a reticle membrane that is transmissive to charged particles in the illumination beam without scattering the charged particles. As in the first representative embodiment, the reticle comprises a black subfield and a white subfield each sized similarly to an exposure unit of the reticle. The reticle is placed at an axial location at which the black and white subfields can be individually illuminated by the illumination beam in a manner similarly to illumination of an exposure unit of the reticle by the illumination beam. A contrast aperture, serving to block passage therethrough of charged particles of the patterned beam scattered by passage through the reticle, is placed substantially at a beam-convergence plane of the projection lens. (Such a plane is also the Fourier plane of a plane defined by the reticle.) The contrast aperture prevents the scattered particles from propagating to the sensitive substrate. The white and black subfields are selectively illuminated individually as required with the illumination beam to align components of the illumination-optical system and projection-lens system.
The xe2x80x9cwhite subfieldxe2x80x9d referred to above is a subfield in which, when illuminated by the illumination beam, the illumination beam passes through the entire subfield without being significantly scattered. Thus, in a scattering-stencil reticle, a white subfield is a completely cut-out subfield. In a scattering-membrane reticle, a white subfield consists only of the reticle membrane and lacks any regions including a layer of a highly scattering material. The xe2x80x9cblack subfieldxe2x80x9d referred to above is a subfield in which, when illuminated by the illumination beam, the illumination beam is intensely scattered during passage through the subfield. Thus, in a scattering-stencil reticle, a black subfield lacks cutouts. In a scattering-membrane reticle, a black subfield consists of the reticle membrane completely overlaid with a layer of the highly scattering material. Optical axial alignments and adjustments are performed while directing the illumination beam to be incident individually on the white subfield and the black subfield.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.